Abstract
We report the passive phase locking of four high power Yb-doped fiber amplifiers with ring cavity. The interference patterns at different output power are observed and the Strehl ratios are measured. The maximum coherent output power of the fiber array is up to 1062 W by multi-stage amplification. The stable beam profiles of various phase relationships are observed by controlling the position of the feedback fiber, in good agreement with the calculated results. By using master oscillator power-amplifier (MOPA) architecture and broadband operation of passively phased systems, higher power scaling with high beam quality appears to be feasible.